Always use alkaline batteries
Never use rechargeable batteries
Left column both batteries + pointing down
Right column both batteries + pointing up
"Batteries low" sign means 1 month battery power left
(Dry solder joints) - re soldering the batteries spring connection terminals to the circuit board has be known to solve the problem.
Thanks. As an electronics engineer anyway I did notice the battery compartment is particularly poorly designed but there are no dry joints and I don't put rechargeables in things like this as they rarely provide enough terminal voltage, so sadly it looks like it's just one-of-those-things. It wouldn't be so bad if they could supply a mains powered version of the controller, no-one really needs it to be portable.
the batteries normally last about 3 years. high battery usage is normally because it is struggling to communicate with the receiver on the boiler. Make sure you don't have stuff in front of the boiler. ( only a door etc)